Conventionally, gamma adjustment and brightness adjustment in correspondence with an input picture signal have been available in an image display apparatus. The gamma adjustment adjusts an input signal-output brightness property (a variation in an output brightness to a variation in an input signal; it is called a gamma property) of an image display apparatus. By such adjustments to control brightness, a nuance of color (chromaticity) and a contrast ratio of an output image, an image substantially equal to an inputted original image can be displayed. In addition, it is also possible to freely control an image quality; for example, an image which is subject to a contrast ratio control with respect to an inputted original image can be displayed.
A gamma adjustment technique in connection with a liquid crystal display apparatus is disclosed in Japanese Unexamined Patent Publication No. 126648/1998 (Tokukaihei 10-126648 published on May 15, 1998). A gamma adjustment circuit disclosed in the publication 10-126648 converts an input analog picture signal into a digital signal by an AD (analog-digital) converter so as to adjust gamma, where the input analog picture signal is amplified at an amplification degree with is selected in accordance with a width (range of an analog input voltage) of the AD converter, thereafter converting it into an digital signal by the AD converter, then, gamma adjustment is performed in accordance with a gamma adjustment property which is determined in relation to selected information of the amplification degree. With this arrangement, it is possible to perform gamma adjustment in a broad range and with high accuracy in conversion processing of a small number of bits, thereby reducing the cost of a circuit necessary for performing gamma adjustment in a broad range and with high accuracy.
Further, Japanese Unexamined Patent Publication No. 64037/1993 (Tokukaihei 5-64037 published on Mar. 12, 1993) discloses a gamma adjustment circuit to perform appropriate gamma adjustment processing with respect to signals of colors R (red), G (green) and B (blue), by obtaining, based on a transmissivity property measured by a brightnessmeter, an appropriate gamma adjustment curve for a liquid crystal display apparatus so as to have a linear function representing an input voltage-output brightness property.
Furthermore, Japanese Unexamined Patent Publication No. 145942/1993 (Tokukaihei 5-145942 published on Jun. 11, 1993) discloses a gamma adjustment technique in connection with a CRT (Cathode Ray Tube) display device, where color control is performed together with gamma adjustment, by measuring gamma properties of colors R, G and B of a CRT display device so as to equalize values of a brightness ratio of the levels of RGB signals, using the measured gamma properties.
Further, Japanese Examined Patent Publication No. 109456/1995 (Tokukohei 7-109456 published on Nov. 22, 1995; corresponding to Japanese Unexamined Patent Publication No. 158416/1989 that is Tokukaihei 1-158416 published on Jun. 21, 1989) discloses a brightness control technique for a color light source in a liquid crystal display, where, in order to prevent an image with the unbalanced look of colors due to reduction in spectral luminous efficiency of each of colors R, G and B in low light, there are provided a first brightness control means for controlling brightness level of an emission element group for each of the colors R, G and B, and a second brightness control means for controlling a balance of brightness levels among a green-color emission element group, a red-color emission element group and a blue-color emission element group, in accordance with the brightness level controlled by the first brightness control means.
Conventionally, the CRT display device is generally arranged to receive a picture signal subject to an inverse gamma compensation. The inverse gamma compensation is such that, in order to attain direct proportionality between an intensity of light in an original image and brightness of each pixel in an image display apparatus, a picture signal is adjusted employing an inverse function of a non-linear function representing an input signal-output brightness property (gamma property) of an image display apparatus. A non-linear function representing a gamma property of the CRT display device can be shown in approximate representation by an exponential function, where a value of the exponential function called a gamma value is generally about 2.2 or so. Accordingly, to the CRT display device is inputted a picture signal subject to the inverse gamma compensation, under the condition that a gamma value is generally 2.2.
Using two types of CRT display devices D and E by different manufacturers, which are commercially available, variations in a gamma value γ(G) and maximum output brightness imax(G) with respect to an average input signal level of brightness G (average signal level of brightness of an input picture signal on a whole screen) are measured, results of which are shown in FIG. 10. Note that, among curves shown in FIG. 10, a curve γD represents a gamma value γ(G) of one type of the CRT display device D, a curve γE represents a gamma value γ(G) of the other type of the CRT display device E, a curve imaxD represents maximum output brightness imax(G) of the CRT display device D, a curve imaxE represents maximum output brightness imax(G) of the CRT display device E. In addition, the average input signal level of brightness G is a relative value whose maximum value is 100%, and the maximum output brightness is a value normalized to have a maximum value of 1.
As is clear from measurement data shown in FIG. 10, in the majority of input images having the average input signal level of brightness G within a fixed range, the maximum output brightness imax(G) and the gamma value γ(G) are substantially constant; on the other hand, in an input image having the average input signal level of brightness G which is out of the range, the maximum output brightness imax(G) decreases.
According to FIG. 10, when the average input signal level of brightness G is about 60%, the actually measured gamma value γ(G) is also about 2.2. Consequently, in that case, the CRT display device accurately reproduces an image in which an inputted picture signal is linearly processed, that is, an original image (picked up image) prior to the inverse gamma compensation.
However, when the average input signal level of brightness G is less than about 60%, or when it is more than about 60%, the gamma value γ(G) of an image output of the CRT display device does not show 2.2, which results in incomplete linear processing, thus failing to obtain an accurate reproduction of an original image (picked up image) on a display.
However, the inventors of the present application have examined and revealed that with such a display property, as shown in FIG. 12, when the average input signal level of brightness is low, an input signal-output brightness property has output brightness in a dark portion which has relatively been increased, thereby attaining an advantage in display, that is, an improvement in visibility in the dark portion.
Further, the inventors of the present invention have examined and revealed that, as shown in FIG. 13, with the foregoing display property, when the average input signal level of brightness is high, the input signal-output brightness property becomes such that output brightness in a bright portion is relatively reduced, and output brightness in an entire screen is relatively reduced. Therefore, in the bright portion are prevented bleached-looking display and glare, thereby improving visibility.
It is commonly unrecognized that the foregoing display property of the CRT display device improves visibility of an image on display. Note that, not all the CRT display devices show this display property, but it is common in a CRT display device to show the display property like this because it has a circuit (automatic brightness limiting circuit) for limiting an increase in a driving current in accordance with an increase in brightness on display, so as to prevent a CRT from being burnt due to the increase in a driving current that is typical of the CRT display device.
On the other hand, in a display apparatus including an emission element such as a backlight, and a light switching element such as a liquid crystal panel, for example, in a liquid crystal display device, when reproducing a picture signal in the display apparatus, the maximum brightness of an image on display is determined substantially according to output of an emission element, and an input signal-output brightness property is determined substantially according to a characteristic of a light switching element. Here, the maximum brightness of an image on display and the input signal-output brightness property are the properties independent from each other. Moreover, in such a display apparatus, as is clear from FIG. 11 showing measurement results of a liquid crystal display device, the maximum output brightness imax(G) of an image on display and an exponential value (a gamma value) γ(G) in which a non-linear function representing the input signal-output brightness property is approximately represented by an exponential function are constant, regardless of an average input signal level of brightness G of an input signal (it is substantially equal to an input signal level of brightness H in a background).
Here, according to the inventors' subjective comparison between the display property (brightness property) of the foregoing consumer-oriented CRT display device and the display property (brightness property) of the foregoing liquid crystal display device, it has been confirmed that such a display property (brightness property) as in the CRT display device is more desirable in terms of a natural image quality.
FIG. 11 shows an input signal-output brightness property obtained as a result of an inverse gamma compensation of an input picture signal and a compensation of a voltage-optical conversion property of a liquid crystal (compensation for a deviation from a linear property), that were performed by a signal processing circuit in the liquid crystal display device.
When inputting a picture signal which is subject to an inverse gamma compensation, such as a picture signal transmitted from a TV broadcast station, to a liquid crystal display device having an input signal-output brightness property as shown in FIG. 11, an output brightness property of an image reproduced on a display surface of the liquid crystal display device becomes the one as shown in FIG. 14.
With this output brightness property, the normalized brightness becomes much higher than a linear property in a portion where brightness is high, i.e., where a normalized signal level of brightness is not less than 0.4. With this output brightness property is obtained an image which gives the impression that a whole image looks bleached and out of focus when viewed, thus failing to properly reproduce an input image.
Furthermore, with an output brightness property of an image reproduced as shown in FIG. 11, when displaying an image having high average brightness, brightness as a whole becomes high. Therefore, a viewer feels that a whole screen is so glaring that he or she cannot fully recognize a slight difference in brightness in a bright portion, thereby being given the impression that the screen is apparently in a state of whiteout. Further, on the contrary, when displaying a dark image having low average brightness, though a dark portion is reproduced substantially according to the linear property, yet a screen is entirely dark, thereby giving a viewer the impression that visibility in the dark portion is insufficient.
On the other hand, in a CRT display device, maximum output brightness is relatively high when average brightness is low, thereby giving the impression that visibility in a dark portion is relatively favorable. Further, when displaying an entirely bright image having high average brightness, maximum output brightness becomes relatively low, thereby suppressing glare and slightly improving visibility of a whole screen.
Such deficiencies in visibility and color tone, such as a bleach-looking screen, due to a display property are pronounced among a CRT display device, a flat display device including a liquid crystal display and a plasma display, and a projection-type display device.
Meanwhile, the foregoing conventional gamma compensation technique has a presupposition that a gamma property of a display apparatus is invariable regardless of a type of an image, and therefore, compensation is performed at the same setting value (gamma value) with respect to any images. This prevents compensation of a deficiency in visibility in the liquid crystal display device as above.
Further, since the foregoing conventional brightness control technique concerns an output adjustment to a color light source, a level of a picture signal to be inputted to a liquid crystal display has not been considered at all. Therefore, this technique also cannot compensate for a deficiency in visibility in the liquid crystal display device as above.
Essentially, a minimum requirement for accurate reproduction of a display image from an input signal is the ability to show a substantially linear input signal-output brightness property on an image display surface of a display apparatus. Further, in order to attain a picture which looks natural to a viewer, one feasible arrangement is such that an I/O property such as a brightness property and a color-tone property of image reproduction can be adjusted arbitrarily; however, with this arrangement, there arises problems such as a complication to an arrangement of signal processing circuitry in an image display apparatus, and an increase in cost.